This invention relates generally to a system for determining the location of the liquid-vapor interface of a liquid stored in an inaccessible container, and specifically to an accurate, remote indicating system utilizing a step motor and command control circuitry with a pair of electronic liquid-vapor interface sensing probes connected to a digital remote display for determining accurately and continuously the liquid level for liquids such as liquified natural gas.
Many liquid level measuring systems are shown in the prior art for determining the level of a liquid stored in a generally inaccessible area utilizing electrical and/or mechanical type probes. Many of these devices have been found unacceptable because of their inherent complexity which often results in errors. Further, servicing and replacement of the units often require emptying the tanks for access to the particular level determining components. Many liquids, such as liquid natural gas for commercial use, must be, of necessity, stored in tanks which cannot be readily inspected internally by a human operator. Liquid natural gas is stored in a closed tank at a temperature of -258.degree. Fahrenheit. One patent issued to Droin, et al, U.S. Pat. No. 2,683,371, issued July 13, 1954 shows a liquid level indicator using capacitative elements in conjunction with a motor driven screw and housing which are immersed within a tank, with control circuitry for an AC motor utilized with the capacitative elements in the circuit logic. The first drawback of the Droin device is that it is inherently inaccurate which can be an extremely important factor when measuring the liquid level in tanks that are extremely large (i.e. many liquid natural gas tanks can be up to 200 feet in diameter). The inherent inaccuracy results from the type of electromechanical drive system in conjunction with capacitative sensors and a mechanical meter for indicating liquid level. Another drawback of the Droin device is the permanent mounting of the probe support housing within the tank making maintenance and servicing extremely costly. Additional drawbacks in the Droin device are that it employs four probe elements and gearing and screw movement, all of which are inherently inaccurate.
The instant invention overcomes the problems shown in the prior art by utilizing a liquid-vapor interface probe having two electronic circuit elements whose resistance varies as a function of element thermal dissipation mounted on the end of a movable cable suspended within the storage tank, the output of each circuit element being connected electrically to a DC stepping motor and electrical control circuitry which includes a system logic such that the resistance of the sensing elements will provide a voltage that triggers stepping pulses to the motor which are coordinated with a pulse counter and digital display to provide extremely accurate measurement (referenced to the bottom of the storage tank) of the liquid-vapor interface within the inaccessible tank. One of the advantages of the present invention is that the interface sensing probe and control cable can easily and quickly be removed from the tank for servicing or repair. Another advantage of the present invention is the fact that the stepping motor, take-up reel, and cables may be easily installed on a tank and are assessibly located on the top of the storage tank, with the cable and interface probe being disposed downward into the tank through a sealed access valve. The present invention is further characterized by having reduced complexity and increased system reliability in addition to increased system accuracy.